Vehicle fuel filler assemblies are known wherein the cap, once screwed onto the filler, is now locked by a key-operated locking device, e.g. using the ignition key of the vehicle itself, the key can be withdrawn from the lock cylinder (inside the cap) even when the cap is not fitted or is fitted improperly to the filler, so that danger exists when the vehicle is started up with a badly fitted or no cap, at all, resulting in possible leakage of fuel vapours or even actual fuel.